Sadistic is the adjective. A sadistic person, or a sadistic act, is cruel — and the cruelty is not only “I don’t care.” It is cruelty that someone enjoys. A sadistic remark is said because it will hurt; a sadistic scene in a film shows pain as something the cruel person likes. People also stretch the word in everyday talk: a hard gym class or a brutal exam might be called sadistic, meaning “this is too much; it feels as if you are enjoying our pain.” That stretch is common, though it is looser and often half-joking. The word is fairly strong and a little formal; in kind speech you would not throw it around about a friend.